Field of the Invention
This invention relates to a radiation image storage panel and more particularly, to a radiation image storage panel comprising a support and a phosphor layer provided thereon which comprises a binder and a stimulable phosphor dispersed therein.
Description of Prior Arts
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and a radiographic intensifying screen.
As a method replacing the above-described radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for example, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (stimulable phosphor sheet) is employed, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; exciting the stimulable phosphor with an electromagnetic wave such as visible light and infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the stimulable phosphor as light emission (stimulated emission); photoelectrically converting the emitted light to give electric signals; and reproducing the electric signals as a visible image on a recording material such as a photosensitive film or on a displaying device such as CRT.
In the above-described radiation image recording and reproducing method, a radiation image can be obtained with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the case of using the conventional radiography. Accordingly, this radiation image recording and reproducing method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (stimulated emission) when excited with stimulating rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or having radiation from an object is absorbed by the phosphor layer of the radiation image storage panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the radiation image storage panel in the form of a radiation energy-stored image (latent image). The radiation energy-stored image can be released as stimulated emission (light emission) by applying stimulating rays to the panel, for instance, by scanning the panel with stimulating rays. The stimulated emission is then photoelectrically converted to electric signals, so as to produce a visible image from the radiation energy-stored image.
It is desired for the radiation image storage panel employed in the radiation image recording and reproducing method to have a high sensitivity and to provide an image of high quality (high sharpness, high graininess, etc.).
In the art of enhancing the above-described quality of image, particularly sharpness, a variety of radiation image storage panels have been developed, for instance, a radiation image storage panel having a phosphor layer of reduced thickness and a radiation image storage panel a part of which is colored. However, these radiation image storage panels have a tendency to cause deterioration of the graininess of images provided thereby. Accordingly, a radiation image storage panel capable of giving an image improved in the graininess as well as the sharpness is desired.
As a method of enhancing both the sharpness and graininess in the radiation image storage panel, adjustment of particle size of a stimulable phosphor employed in the panel has been proposed. More in detail, the enhancement in both the sharpness and graininess of the image can be obtained by employing a stimulable phosphor having small particle size for formation of the phosphor layer of the panel.
Concerning the above-described method, the present applicant has already applied for patent an invention on a radiation image storage panel characterized in that a stimulable phosphor employed in a phosphor layer of the panel has such a particle size distribution that phosphor particles having a size (diameter) of not less than 100 .mu.m are present in an amount of not more than 1% by weight and phosphor particles having a size of not less than 1 .mu.m are present in an amount of not less than 50% by weight (Japanese Patent Application No. 57(1982)-65609, corresponding to U.S. Pat. No. 4,547,672 and European Patent Publication No. 83103790.8).
However, it is not easy to adjust the particle size of a stimulable phosphor employed in the phosphor layer of the panel to be included within a certain range so as to give the disired sharpness and graininess of an image provided by the panel. This is because the particle size of the stimulable phosphor easily varies depending upon the conditions of preparation thereof, so that it is difficult to adjust the particle size of the phosphor to a desired level in the stage of the preparation. Otherwise, it is also difficult to so adjust the particle size of the resultant stimulable phosphor by means of classification and the like as to give the desired quality of the image provided by the panel. In addition, this is accompanied by complicated procedures and decrease of phosphor yield.